A variety of iron oxides have found use as pigments, catalysts, ferrites and as components in magnetic recording media. The preparation of iron oxide particles is an extensively developed, but still highly empirical, art. An illuminating commentary on the complexities involved has been provided by T. Takada (Denki Kagaku Oyobi Kogyo Butsuri Kagaku, 37 (5), pp. 328-335 (1969)). These complexities presumably account for the publication of hundreds of papers and patents on the hydrolysis of iron salts and iron oxide preparative processes.
The processes and products disclosed in the following patents and papers are the closest art to the present invention known to applicant.
U.S. Pat. No. 2,416,744 describes the preparation of orange-red, iron oxide particles of undisclosed structure by hydrolysis of oxidized ferrous sulfate solutions with a base, such as NH.sub.4 OH or LiOH, at a pH of 6 to 11 and at a temperature of 60.degree.-75.degree. C. Only pigment utility is taught.
U.S. Pat. No. 2,558,302 teaches the preparation of needle-shaped, iron oxide crystals, less than 0.1 micron in diameter, by the base hydrolysis of ferric salts under strongly alkaline conditions at 40.degree.-95.degree. C. A dependency of non-agglomerating character on a combined anion content of less than 1% is taught. No further details of structure or composition are given.
U.S. Pat. No. 2,866,686 discloses the preparation of acicular alpha iron oxide particles about 0.25 to 1.5 microns long, 0.1 to 0.3 microns wide and having a cubic crystal lattice. One equivalent of ferrous sulfate is reacted with 0.87 equivalents of sodium hydroxide and the resulting precipitate oxidized at about 60.degree. for up to 4 hours. The acicular particles are convertible to magnetic iron oxides having coercive forces of from 220-360 oersteds. Nothing more is disclosed as to fine structure of any of the oxide particles.
U.S. Pat. No. 3,009,821 describes the hydrolytic preparation of mixtures of rhombohedral and acicular hematite particles by the reaction of ferrous sulfate and alkali hydroxide under oxidizing conditions at 50.degree.-100.degree. C. and at a pH of 4 or less in the presence of a colloidal iron oxide seed. A dependency of particle acicularity on the iron concentration and reactant ratio employed for seed preparation is taught. The maximum dimensions of the hematite particles vary from "well under" 0.2 micron to "somewhat larger" than 0.2 micron.
U.S. Pat. No. 3,082,067 discloses the preparation of prismatic gamma ferric hydroxide (hematite) particles by reacting ferrous sulfate with sodium hydroxide at 0.degree.-35.degree. C. and oxidizing the resultant slurry at 55.degree.-60.degree. C. Dehydration of the hematite particles gives isometric, prismatic crystals of "ferromagnetic gamma ferric oxide".
British Pat. No. 656,265 discloses the growth of hematite-containing iron oxide particles of greater than colloidal dimensions by "agglomeration" of smaller particles while heating ferric hydroxide in the presence of ferrous and cupric or zinc ions at temperatures of 150.degree. or higher with minimal agitation. The ferric hydroxide may be preformed by reaction of ferric sulfate with excess alkali hydroxide. No further disclosure as to crystal structure, shape or size is provided.
S. Nobuoka (Kogyo Kagaku Zasshi 68 (12), 2311-17 (1965); C.A. 65 6407g) examined, by X-ray diffraction and infrared absorption, the precipitates formed in the reaction between ferric sulfate and sodium hydroxide in aqueous solutions. The precipitate formed at pH 5-6 is an amorphous basic ferric sulfate which can be converted to cubic hematite crystals by hydrothermal treatment at 120.degree.-130.degree. C. At pH 12-13, amorphous Fe(OH).sub.3 precipitates and then gradually alters to "needles" of alpha-(FeOOH). The latter product can be converted to alpha-Fe.sub. 2 O.sub.3 (hematite) by hydrothermal treatment at 200.degree.-300.degree. C.
C. J. Haigh (Australasian Inst. of Mining and Metallurgy, Proc. 223, 49-56 (1967); C.A. 68 32151v) obtained Fe.sub.2 O.sub.3 from acidic solutions of pure ferric sulfate at 200.degree. C. and from similar solutions also containing Na.sub.2 SO.sub.4, Na.sub.2 CO.sub.3, NH.sub.4 OH or K.sub.2 SO.sub.4 at temperatures from 180.degree.-220.degree. C.
Takada (loc. cit.) discloses the preparation of iron compounds having jarosite structures in acidic iron sulfate solutions containing alkali metal ions. In acid solution, the type of product precipitated depends on temperature and the type of anion present. A reference is given in Table III of this paper to the reaction of FeCl.sub.3 at 150.degree. C. with more than three molecular proportions of LiOH to produce a lithium ferrite.
It is evident from the foregoing publications that a number of interdependent variables are determinative of the types of products which are obtained by the hydrolysis of iron salts. The properties of a given iron oxide product are determined by the shape, size, fine structure and chemical composition of the particles of which it is composed. In order to obtain particles of a specific type, a narrowly specified set of reaction parameters must be maintained.